A lithium-ion secondary battery includes therein a cathode and an anode which are separated by a separator that is in a form of film and is porous. A process of producing the separator includes a washing step of removing an unnecessary substance from the film which has been prepared in advance.
For example, if not limited to a separator, techniques disclosed in Patent Literatures 1 and 2 are known as techniques to wash a sheet or a film. Patent Literature 1 discloses a washing tank including two tanks for roughly and thoroughly washing a heat-sealing multilayer sheet in sequence. Patent Literature 2 discloses a washing section having a plurality of stages for soak-washing and spray-washing an optical plastic film in sequence.